Significant, as in "statistically significant", usually that you've got sufficient evidence to conclude that your result is not simply due to random chance.
Substantial means that your result is big enough to have any practical import. In other words, is there any substance to the result?
For an example of a result that is significant but not substantial, suppose you find after surveying millions of people that members of demographic group X score 1/10 point higher than average on an IQ test.
For an example of a result that is substantial but not significant, sales figures are often so variable that it's impossible to determine with confidence whether even a 100% jump in revenues is due to a recent ad campaign or just a random fluke.
Substantial means that your result is big enough to have any practical import. In other words, is there any substance to the result?
For an example of a result that is significant but not substantial, suppose you find after surveying millions of people that members of demographic group X score 1/10 point higher than average on an IQ test.
For an example of a result that is substantial but not significant, sales figures are often so variable that it's impossible to determine with confidence whether even a 100% jump in revenues is due to a recent ad campaign or just a random fluke.